A fish disease called saprolegniasis has long been a problem in farms of freshwater fish such as rainbow trout. Saprolegniasis is caused by species belonging to genera Saprolegnia, Achlya, and Aphanomyces of family Saprolegniaceae, order Saprolegniales, class Oomycetes. Recent molecular analysis and biochemical research has confirmed that oomycetes are classified as Heterokontae in the kingdom Protista, that oomycetes have a fungus-like appearance (Non-patent Literature (NPL) 1), and that the genus Phytophthora, which is plant-pathogenic, also belongs to this group.
Malachite green dye, which is active even at low concentrations against saprolegniasis-causing organisms (NPL 2) and is inexpensive, has been used as a prophylactic and therapeutic agent. In recent years, however, due to concerns about its carcinogenicity, malachite green has been banned for use with farmed edible fish. As a substitute saprolegniasis prophylactic medication for farmed fish or fish eggs, ozone (Patent Literature (PTL) 1), electrolyzed water (PTL 2), organic acids (PTL 3), Bacillus subtilis strains (PTL 4), etc., have been proposed. Further, medications such as Pyceze (trade name, a product of Novartis Animal Health K.K.), which comprises the synthetic antimicrobial preservative bronopol (C3H6BrNO4) as an active ingredient, are commercially available. However, Pyceze is expensive compared to malachite green, and the active ingredient bronopol has observable potent toxicity in aquatic organisms such as edible oysters (EC50 0.77 mg/L), water fleas (EC50 1.4 mg/L), which are useful as fish feed, and green algae (EC50 0.0537 mg/L) (NPL 3). Therefore, Pyceze has problems such as requiring dilution with a large amount of water upon disposal. Accordingly, there is a need for the development of a safer and effective antifungal agent.